This invention relates generally to shift registers and particularly to a shift register useful as a select line scanner for liquid crystal displays.
Liquid crystal television and computer displays (LCDs) are known in the art. For example, see U.S. Pat. Nos. 4,742,346 and 4,766,430, both issued to G. G. Gillette et al. and incorporated herein by reference. Displays of the type described in the Gillette patents include a matrix of liquid crystal cells which are arranged at the crossovers of data lines and select lines. The select lines are sequentially selected by a select line scanner to produce the horizontal lines of the display. The data lines apply the brightness (gray scale) signals to the columns of liquid crystal cells as the select lines are sequentially selected. Each liquid crystal cell is associated with a switching device through which a ramp voltage is applied to the liquid crystal cells in the selected line. Each of the switching devices is held on by a comparator, or a counter, which receives the brightness signal to permit the ramp voltage to charge the associated liquid crystal cell to a voltage proportional to the brightness level received by the comparator from the data line.
Preferably, the drive circuitry, which drives the data lines, and the select line scanner, which selects the horizontal lines to be displayed, are fabricated directly onto the same substrate as the liquid crystal cells at the same time that the switching devices are fabricated. Also, because a large number of data lines and select lines are required for a television or computer display, and because the small pixel pitch limits the space available for laying out the driver circuitry, it is essential to keep the circuitry as simple as possible in order to obtain high manufacturing yield. It can also be advantageous to utilize solid state devices of the same conductivity type throughout the display device. For these reasons, there is a need for a simple, reliable select line scanner for use in liquid crystal displays. The invention fulfills this need by a provision of a multistage shift register wherein each stage is comprised of a minimum number of solid state devices which, preferably, are thin film transistors (TFT's), and all of which can be of the same conductivity type.